Well I can’t tell you the exact gene would need to be changed, there are plenty of animals able to deal with the salinity of sea water. Basically the kidney has a certain ability to concentrate salt. The greater the ability the saltier the animal’s urine is and the saltier water can be consumed.
If I am remembering correctly the threshold for salinity of water that can be consumed is up to a level just below that of the threshold of the kidney’s ability to concentrate salt.
So we would not need “super kidneys” just normal ones similar to those of one of many animals in the sea. But we might have a shortened kidney lifespan because of it.